Multiscale all-soft electronic devices and circuits based on liquid metal

ABSTRACT

In a method making a flexible electrical conductor, a mask layer (216) is applied to a substrate (210). A portion of the mask layer (216) is removed to expose the substrate (210) in an exposed shape (220) corresponding to the conductor. A liquid phase conductor (232) is applied to the portion of the substrate (210). The mask layer (216) is dissolved with a solvent (238) to leave a shaped liquid phase conductor (234) corresponding to the exposed shape on the substrate (210). A primary elastomer layer (240) is applied onto the substrate (210) and the shaped liquid phase conductor (234). The primary elastomer layer (240) and the shaped liquid phase conductor (234) are removed from the substrate (210). A secondary elastomer layer (242) is applied to the shaped liquid phase conductor (234) and the primary elastomer layer (240) to seal the shaped liquid phase conductor (234) therein.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a divisional and claims the benefit of U.S. patentapplication Ser. No. 16/875,572, filed on May 15, 2020, which claims thebenefit of U.S. Provisional Patent Application Ser. No. 62/849,372,filed May 17, 2019; this application also claims the benefit of U.S.Provisional Patent Application Ser. No. 62/864,571, filed Jun. 21, 2019,the entirety of each of which is hereby incorporated herein byreference.

STATEMENT OF GOVERNMENT SUPPORT

This invention was made with government support under 1542174, awardedby the National Science Foundation. The government has certain rights inthe invention.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to electronic circuits and, morespecifically, to electronic circuits with stretchable passive elementsand circuits.

2. Description of the Related Art

Progress in soft functional material synthesis and manufacturingtechnology has enabled bioinspired and skin-like soft electronics forapplications ranging from entertainment to healthcare. Unlikeconventional solid-state electronics, soft electronics can belightweight, stretchable, and reconfigurable, with biocompatiblecharacteristics for skin-mountable and wearable sensing electronics.Flexible and stretchable characteristics can be achieved by using either2D or 3D compliant wave-like, solid metal patterns or elastic conductorsbased on conductive nanomaterials embedded in a polymer matrix.

An alternative approach to realize all-soft microsystems is the use ofintrinsically soft conductors, such as gallium-based liquid metal suchas eutectic gallium-indium alloy (EGaIn), which is a liquid phaseconductor at room temperature. The use of EGaIn in flexible electronicscan be desirable due to its nontoxicity, mechanical stability (virtuallyunlimited stretchability), thermal conductivity and electricalconductivity. The low melting temperature and negligible vapor pressureof EGaIn facilitate room-temperature and ambient pressure manufacturingprocessing. Moreover, thanks to the formation of a thin oxide layer onthe EGaIn surface under atmospheric oxygen level, EGaIn structuresmaintain their mechanical shapes, allowing formation of EGaIn patternson soft elastomeric substrates.

For interfacing with individual cells, the ability to patternsubmicrometer metallic structures embedded in soft substrates is ofsignificant interest. Considering the size of a single biological cell,such as platelets with a diameter of 2-3 μm, mechano-transducers shouldbe manufactured with submicron scale features and soft, biomimeticproperties. Existing fabrication technologies, including the transferprinting of compliant solid metal patterns, nanoprinting, directprinting of nanomaterials, and EGaIn patterning, are currently notsuitable to fabricate such soft and stretchable electronic devices withsubmicron-scale resolution.

Certain existing EGaIn circuit production methods place EGaIn in achannel defined by an elastomer. The channel can be shaped in the formof passive electronic components. Unfortunately, existing processes havedifficulty making wires that have a width greater than 500 μm. Suchwidths can be useful in power distribution networks, ground planes andother circuit elements.

Therefore, there is a need for flexible circuit elements and a methodfor making such elements employing soft conductors.

SUMMARY OF THE INVENTION

The disadvantages of the prior art are overcome by the present inventionwhich, in one aspect, is a method of making a flexible electricalelement configured to conduct electricity while at a normal operatingtemperature, in which a stamp is generated so as to have a shapedsurface corresponding to the flexible electrical element. A liquid phaseconductor is applied to the shaped surface. The liquid phase conductorincludes a material that remains liquid while at the normal operatingtemperature of the flexible electrical element. The stamp is stampedonto a receptive surface of a primary elastomer layer. The receptivesurface has an aspect that causes the liquid phase conductor from theshaped surface to remain on the receptive surface of the primaryelastomer layer when the stamp is removed therefrom. A secondaryelastomer layer is applied to the elastomer substrate so as to seal theliquid phase conductor onto the elastomer substrate.

In another aspect, the invention is a method making a flexibleelectrical conductor configured to conduct electricity while at a normaloperating temperature, in which a mask layer is applied to a substrate.A portion of the mask layer is removed so as to expose a portion of thesubstrate in an exposed shape corresponding to the flexible electricalconductor. A liquid phase conductor is applied to the portion of thesubstrate. The mask layer is dissolved with a solvent so as to leave ashaped liquid phase conductor corresponding to the exposed shape on thesubstrate. A primary elastomer layer is applied onto the substrate andthe shaped liquid phase conductor. The primary elastomer layer and theshaped liquid phase conductor are removed from the substrate. Asecondary elastomer layer is applied to the shaped liquid phaseconductor and the primary elastomer layer so as to seal the shapedliquid phase conductor therein.

In yet another aspect, the invention is an electronic element thatincludes an elastomer member defining a channel. The channel has alength and a width that are at least 500 μm. A liquid phase conductorfills the channel.

These and other aspects of the invention will become apparent from thefollowing description of the preferred embodiments taken in conjunctionwith the following drawings. As would be obvious to one skilled in theart, many variations and modifications of the invention may be effectedwithout departing from the spirit and scope of the novel concepts of thedisclosure.

BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWINGS

FIGS. 1A-1H include a series of schematic diagrams demonstrating a firstembodiment of a method of making flexible electronic elements.

FIGS. 2A-2K include a series of schematic diagrams demonstrating asecond embodiment of a method of making flexible electronic elements.

FIG. 3 is a schematic diagram of a flexible resistor.

FIGS. 4A-4C are schematic diagrams of a flexible resistor, a flexibleinductor and a flexible capacitor, respectively, made according to onerepresentative embodiment of the invention.

FIG. 5 is a schematic diagram of a flexible electrode array madeaccording to one representative embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment of the invention is now described in detail.Referring to the drawings, like numbers indicate like parts throughoutthe views. Unless otherwise specifically indicated in the disclosurethat follows, the drawings are not necessarily drawn to scale. Thepresent disclosure should in no way be limited to the exemplaryimplementations and techniques illustrated in the drawings and describedbelow. As used in the description herein and throughout the claims, thefollowing terms take the meanings explicitly associated herein, unlessthe context clearly dictates otherwise: the meaning of “a,” “an,” and“the” includes plural reference, the meaning of “in” includes “in” and“on.”

Parylene is a common name for a variety of chemical vapor depositedpoly(p-xylylene) polymers.

As shown in FIGS. 1A-1H, in one embodiment of a method of making aflexible electrical element that is configured to conduct electricitywhile at a normal operating temperature, material is removed from asubstrate 111 using a common lithography method, such as by using ane-beam lithography device 112 so as to form a stamp 110, as shown inFIG. 1A. The stamp 110 is shaped so as to have a surface correspondingto the desired shape of the flexible electrical element. The stamp 110,if made from a flexible elastomer (such as polydimethylsiloxane (PDMS)),can define a narrow channel 114 (e.g., narrower than about 500 μm) thatfollows the path of a desired narrow wire.

As shown in FIG. 1B, a donor layer 120 (which can also include PDMS) iscoated with a liquid phase conductor 122 (such as EGaIn or othereutectic alloy), which remains in a liquid form while at the normaloperating temperature (e.g., room temperature) of the flexibleelectrical element. If EGaIn is used, a roughly 1-3 nm layer 124 ofgallium oxide will form on its top surface. The shaped outer surface ofthe stamp 110 can be coated with a chemical modifier 116 (such as withtoluene) to attract the liquid phase conductor 122 thereto.

As shown in FIG. 1C, the stamp 110 is pressed against the liquid phaseconductor 122, which causes the liquid phase conductor 122 to enter thechannel 114 and adhere to the chemically modified surface 116 of thestamp. (It should be noted that such a narrow channel is not used insome applications, such as ones that generate flexible elements havingwidths of at least about 500 μm.) This stamping action can be repeatedseveral times for more complete adhesion of the liquid phase conductor122 to the stamp 110. As shown in FIGS. 1D-1E, the stamp is then stampedagainst a receptive surface if a primary elastomer layer 130 (e.g., alayer of polydimethylsiloxane). The receptive surface has a treatment132, or other aspect, that causes the liquid phase conductor 122 fromthe shaped surface to remain on the receptive surface. In oneembodiment, the treatment 132 includes a paper texturing which can beachieved by applying micro cellulose fiber to the receptive surface.

When the stamp 110 is removed, as shown in FIG. 1F, shaped portions 128of the liquid phase conductor having a shape corresponding to that ofthe shaped surface of the stamp 110 are left on the surface of theprimary elastomer layer 130. If channel 114 in an elastomer stamp 110 isused to form a liquid phase conductor wire 126, as shown in FIG. 1G,then a secondary elastomer layer 142 (such as a polydimethylsiloxanelayer) is placed against the stamp 110 to encapsulate the wire 126. Aflexible via 144 can be used to connect the liquid phase conductor wire126 to external elements. As shown in FIG. 1H, the shaped portions 128are encapsulated with a secondary elastomer layer 146 (such as apolydimethylsiloxane layer) to seal the shaped portions 128 to theprimary elastomer layer substrate 130, thereby making a flexible circuitelement having the desired shape.

In another embodiment, as shown in FIGS. 2A-2K, a mask layer 216 (suchas a poly(methyl methacrylate) layer) is applied to a substrate, whichcan include a rigid layer 210 (such as silicon or SiO₂ layer), ontowhich is applied a sacrificial layer 212 (such as poly(acrylic acid)(PAA)) and a separation layer 214 (such as parylene C), as shown in FIG.2A. As shown in FIG. 2B, a portion corresponding to the shape of adesired flexible electrical conductor is removed from the mask layer 216so as to generate an opening 220 in the desired shape of the conductorthat exposes the sacrificial layer 212. This can be done using aconventional circuit lithography method, such as e-beam lithography 218.As shown in FIG. 2C, an adhesion layer 222 (such as a thin film of Ti/Auat 5 nm to 30 nm in thickness, in one embodiment) having an affinity forthe liquid phase conductor is applied to all exposed surfaces. As shownin FIG. 2D, a liquid phase conductor layer 232 (such as EGaIn) isapplied to a stamp 230 (such as a PDMS layer), which is pressed againstthe adhesion layer 222. When the stamp is removed, as shown in FIG. 2E,the liquid phase conductor layer 232 adheres to the adhesion layer 222.By using a suitable solvent 238 (such as acetone when the mask layercomprises PMMA), as shown in FIG. 2F, the remaining mask layer 216 isremoved, leaving the liquid phase conductor layer 232 in the desiredshape, as shown in FIG. 2G. As shown in FIG. 2H, a primary softelastomer layer 240 (such as PDMS) is applied to the liquid phaseconductor layer 232 and the exposed portions of the separation layer214. As shown in FIG. 2I, the soft elastomer layer 240, the remainingliquid phase conductor layer 232, the separation layer 214 and thesacrificial layer 212 are removed from the rigid layer 210. As shown inFIG. 2J, the separation layer 214 is then etched away so as to exposethe primary soft elastomer layer 240 and liquid phase conductor layer232 (and the trace amount of the remaining adhesion layer 222). As shownin FIG. 2K, the secondary soft elastomer layer 242 (such as PDMS) isapplied so as to encapsulate the liquid phase conductor layer 232. A via244 can also be applied to couple the component to other components.Micro-scale elements (e.g., light emitting diodes, microchips, etc.) canalso be embedded in the elastomer layers to add functionality.

As shown in FIG. 3, a liquid phase conductor 326 can be shaped so as toact as a passive electronic component 300 that is encapsulated in a softelastomer block 310 and that can be electrically coupled to othercomponents with vias 344. While this figure shows only a singlecomponent, many different components can be integrated into a singlesoft elastomer block 310. As shown in FIGS. 4A-4C, such components caninclude, for example, a resistor 410, an inductor 412 and a capacitor414. A complex flexible microelectrode array 500 is shown in FIG. 5, inwhich a first plurality of flexible wires 520 is spaced apart from asecond plurality of flexible wires 522, both of which are encapsulatedin a soft elastomer material 510.

The following paragraphs describe methods employed in a firstexperimental embodiment:

Subtractive Reverse Stamping Technique Based on Soft Lithography: ForPDMS mold preparation, a microfabricated photoresist master on a siliconwafer with critical dimension of 2 μm was fabricated. To create the PDMSmold, liquid PDMS (10:1 ratio of PDMS prepolymer and curing agent,Sylgard 184, Dow Corning) was either drop casted or spin coated on thefabricated silicon master molds and cured at 60° C. for 8 hours. A 500μL droplet of toluene (Toluene, ACS grade, VWR International) wasdrop-coated on a glass substrate and subsequently dried at roomtemperature and under atmospheric pressure for 5-10 minutes. Then, thePDMS mold was placed on the glass substrate for chemical surfacemodification. In the micro-transfer molding process, EGaIn(gallium-indium eutectic, >99.99% trace metal basis, Sigma-Aldrich) wasdispensed and spread using a PDMS roller on a donor PDMS substrate.Afterward, the PDMS mold was gently pressed onto the EGaIn thin film andseparated from it. Unwanted liquid metal residue on the outside of thechannel was transferred to a sacrificial PDMS layer, and this transferprocess was repeated several times (≈15 times) until all residue isremoved. The EGaIn-filled PDMS mold was then bonded to an additionalPDMS layer using either drop casting or spin coating.

Additive Stamping Technique Based on Soft Lithography: For the PDMSstamp preparation, an acrylic master with critical dimension of 500 μmwas fabricated using a CO2 laser cutter, and liquid PDMS (10:1 ratio ofPDMS prepolymer and curing agent, Sylgard 184, Dow Corning) was dropcasted on the fabricated molds and cured at 60° C. for 8 hours. Forpaper-textured PDMS preparation, a small piece of standard printingpaper (Office Depot #348-037) was taped on a flat substrate, and liquidPDMS (10:1 ratio of PDMS prepolymer and curing agent, Sylgard 184, DowCorning) was either drop casted or spin coated on the paper substrate.After curing at 60° C. for 8 hours, the polymerized PDMS was gentlypeeled off from the paper substrate. With this process, the microcellulose fiber structures can be effectively transferred to the PDMSsurface. EGaIn (gallium-indium eutectic, >99.99% trace metal basis,Sigma-Aldrich) was dispensed and spread using a PDMS roller on a donorPDMS substrate. The PDMS stamp was gently pressed onto the EGaIn film,and then the EGaIn film was stamped to the paper-textured PDMSsubstrate. The patterned EGaIn films on the paper-textured PDMS werethen sealed by an additional PDMS layer using either drop casting orspin coating, and commercial copper tape was used for electricalcontacts. All PDMS samples were polymerized at 60° C. for 8 hours.

Another experimental embodiment includes a hybrid lithography process isintroduced that combines electron-beam lithography (EBL) fornano/microstructure fabrication with soft lithography for EGaIntransfer. This hybrid lithography process is applied to a biphasicstructure, including a metallic adhesion layer coated with EGaIn. Thehybrid fabrication approach enables high-resolution and high-densityall-soft electronic devices, including passive electronic components,resistive strain sensor arrays, and microelectrode arrays. Inparticular, EGaIn thin-film patterning with feature sizes as small as180 nm and 1 μm line spacing were demonstrated. The intrinsically softEGaIn structures, patterned by the developed hybrid lithographytechnique, offer a combination of resolution, electrical conductivity,and electronic/wiring density. Thanks to the intrinsically soft EGaInproperties, the fabricated soft devices can endure mechanicaldeformation up to 30%, while maintaining electrical functionality.

In an experimental embodiment, the fabrication process typicallyincludes three fundamental steps: nano/microstructure fabrication usingEBL (or any other lithography technique able to pattern submicrometerfeatures), EGaIn transfer using a stamping process, and soft materialencapsulation and final release from the silicon (Si) carrier wafer. Theprocess starts by spin-coating a water-soluble sacrificial material(poly(acrylic acid), PAA) on a silicon wafer at 2000 rpm for 30 secondsand baking the film at 100° C. for 60 seconds. On top of the PAAsacrificial layer, a 600-nm-thick parylene-C barrier film is depositedby chemical vapor deposition (CVD) in order to protect the underlyingPAA during the subsequent EGaIn patterning as well as while releasingthe fabricated soft electronic devices from the Si wafer after the softmaterial encapsulation. EBL is then used to pattern a spin-coatedpoly(methylmethacrylate) (PMMA) layer with a thickness between 300 nmand 1 μm. After exposure in the EBL tool (Elionix ELS G-100), the PMMAfilm is developed using a mixture of methyl isobutyl ketone (MIBK) andisopropanol with 1:1 ratio. Alternatively, other lithography processeswith submicron resolution can be considered for this step. In the nextstep, a stamping process is used to transfer an EGaIn thin film onto thepatterned PMMA structures. To improve the adhesion and uniformity of thestamped EGaIn on the parylene-C-coated substrate, a biphasic structurewas adopted. To this end, a thin metallic adhesion layer (such as Ti/Au,5 nm/30 nm in thickness) is first deposited using electron-beamevaporation on the patterned PMMA nano/microstructures. The purpose ofthis metallic adhesion layer is to enhance the adhesion and wettingcharacteristics during the EGaIn stamping process while maintainingEGaIn's electrical and mechanical properties. Then, a non-structuredPDMS stamp is wet with EGaIn and gently pressed 2-3 times onto theAu-coated nano/microstructures, transferring a thin EGaIn film whichforms an alloy with the underlying Au adhesion layer. A PMMA lift-offprocess with acetone is then used to pattern the stamped EGaIn on Au.

To highlight the impact of the Au adhesion layer on the EGaInwettability, the EGaIn stamping process was carried out on patternedPMMA structures without and with the use of the Au adhesion layer.Without the adhesion layer, the stamped EGaIn is not uniformly spreadonto the patterned PMMA structure, resulting in non-uniform and roughEGaIn surfaces with EGaIn droplets as well as non-covered areas afterPMMA lift-off. In contrast, by utilizing the Au adhesion layer duringthe EGaIn stamping process, the stamped EGaIn uniformly spreads acrossthe Au film and fills concave nano/micropatterns up to the designed PMMAthickness. The EGaIn stamped on the Au adhesion layer demonstratedstrong adhesion and uniform wetting and, therefore, could besuccessfully patterned using the PMMA lift-off process without anystructural deformation.

Next, the remaining EGaIn structures are covered with a soft elastomer(e.g., poly(dimethyl siloxane), PDMS), and the fabricated devices arereleased from the Si carrier wafer by dissolving the sacrificial PAAlayer in water for at least six hours. Finally, the parylene-C barrierlayer is etched using an oxygen plasma in a reactive-ion etching (RIE)system, and the back side of the soft electronic device is sealed with asoft elastomer. It should be noted that optical lithography with apositive-tone photoresist can be utilized as well for the microstructurefabrication. Moreover, other lithography techniques able to patternsubmicron-scale sacrificial structures, such as direct laser writing 54or 3D nanoprinting, can be potentially utilized for cost-effectivefabrication.

The following paragraphs describe methods employed in a secondexperimental embodiment:

Nano/microstructure fabrication process: A water-soluble sacrificiallayer, poly (acrylic acid) (PAA, Polyscience, Inc.), was spun on a Siwafer at 2000 rpm for 30 s and baked at 100° C. for 60 s, resulting in≈2 μm film thickness. On top of the PAA sacrificial layer, a parylene-Cfilm with 600 nm thickness was deposited by chemical vapor deposition(CVD, SCS Labcoter PDS 2010). For nano/microstructure definition,electron-beam lithography (EBL, Elionix ELS G-100) was utilized topattern spin-coated poly(methylmethacrylate) (PMMA, Micro-Chem Corp.)films with thicknesses ranging from 300 nm to 1 μm. In the EBL process,the samples with different PMMA thicknesses were all exposed using a1-nA current with a proximity effect correction (β=30 and η=0.6). Theapplied dose was adjusted from 400 μC cm-2 to 630 μC cm-2 because of thedifferent PMMA thicknesses. For example, a dose of 510 μC cm-2 wasselected for the 1-μm-thick PMMA film. Then, a thin metallic adhesionlayer, either Ti/Au or Ti/Cu, was deposited onto the PMMAnano/micropatterns using an electron-beam evaporator with a targetthickness of 5 nm/30 nm.

PDMS stamp preparation and EGaIn stamping process: For PDMS stampfabrication, a general replica molding process was used using an acrylicmaster fabricated using a CO2 laser cutter (Hermes LS500XL). Liquid PDMS(10:1 ratio of PDMS pre-polymer and curing agent, Sylgard 184, DowCorning) was drop-casted on the acrylic master and cured at 60° C. for 8h. PDMS stamps with various shapes (e.g. circle or rectangle) and sizes(e.g. 5 mm×5 mm to 30 mm×30 mm) were designed and fabricated to stampEGaIn onto the Au- or Cu-coated nano/microstructures. EGaIn(gallium-indium eutectic, >%99.99 trace metal basis, Sigma-Aldrich) wasdispensed on a donor PDMS substrate using a syringe and spread andflattened by a PDMS roller. In the next step, the fabricated PDMS stampwas wet with EGaIn by pressing it on the EGaIn-coated donor PDMSsubstrate and gently stamped 2-3 times onto the Au- or Cu-coatednano/micropatterns to transfer the EGaIn thin film. The stamped EGaIn onAu or Cu was finally patterned using a PMMA lift-off process withacetone.

Soft material encapsulation and release process: The patterned EGaInstructures were encapsulated with liquid PDMS (10:1 ratio of PDMSpre-polymer and curing agent, Sylgard 184, Dow Corning) either by spincoating or drop casting. The fabricated soft electronic devices werethen released by submerging the samples into water for >6 h. After thePAA sacrificial layer etching was completed, the fabricated soft deviceswere floating on the water surface and could gently be transferred to aglass substrate to etch the parylene-C layer. The parylene-C layer wasetched using an oxygen plasma in a reactive-ion etching system (RIE,Vision 320 RIE) for >7 min or until the parylene-C film was completelyremoved. Under the etching conditions of 200 mTorr pressure and 200 Wpower, the tested parylene-C etch rate using oxygen plasma was ≈100 nmmin−1. After etching the parlylene-C layer, the soft electronic deviceswere encapsulated again with PDMS for backside sealing.

Although specific advantages have been enumerated above, variousembodiments may include some, none, or all of the enumerated advantages.Other technical advantages may become readily apparent to one ofordinary skill in the art after review of the following figures anddescription. It is understood that, although exemplary embodiments areillustrated in the figures and described below, the principles of thepresent disclosure may be implemented using any number of techniques,whether currently known or not. Modifications, additions, or omissionsmay be made to the systems, apparatuses, and methods described hereinwithout departing from the scope of the invention. The components of thesystems and apparatuses may be integrated or separated. The operationsof the systems and apparatuses disclosed herein may be performed bymore, fewer, or other components and the methods described may includemore, fewer, or other steps. Additionally, steps may be performed in anysuitable order. As used in this document, “each” refers to each memberof a set or each member of a subset of a set. It is intended that theclaims and claim elements recited below do not invoke 35 U.S.C. § 112(f)unless the words “means for” or “step for” are explicitly used in theparticular claim. The above described embodiments, while including thepreferred embodiment and the best mode of the invention known to theinventor at the time of filing, are given as illustrative examples only.It will be readily appreciated that many deviations may be made from thespecific embodiments disclosed in this specification without departingfrom the spirit and scope of the invention. Accordingly, the scope ofthe invention is to be determined by the claims below rather than beinglimited to the specifically described embodiments above.

What is claimed is:
 1. An electronic element, comprising: (a) an elastomer member defining a channel, in which the channel has a length and a width that are at least 500 μm; and (b) a liquid phase conductor filling the channel.
 2. The electronic element of claim 1, wherein the elastomer member comprises polydimethylsiloxane.
 3. The electronic element of claim 1,wherein the liquid phase conductor comprises a eutectic alloy.
 4. The electronic element of claim 3, wherein the eutectic alloy comprises a eutectic gallium-indium alloy. 